I'm currently using a Nexus S, and had a Nexus One before that, and I don't see any difference in the responsiveness or smoothness of the UI compared to my wife's iPhone 4.

I am willing to bitch about sometimes choppy widget refreshing on my Galaxy Tab 10.1, but given that the iPad doesn't even have widgets, I'm not sure you can compare. As for app performance, it's fast any clean with any decently written app (which is most of them).